This invention relates to material analysis apparatus, and more particularly to apparatus for measuring the contents of a bulk commodity, such as a grain. The description will be facilitated by addressing the specific problem of measuring the moisture content of a farm grain, such as corn, soybeans or the like.
In measuring the moisture content of a grain, it is known to use a test cell that comprises a capacitor in which the grain sample is introduced and to obtain a reading representative of a moisture content based upon an electrical measurement of the grain filled capacitor. In the past, such measurements have been made by connecting the test cell as a capacitor in the tank circuit of an oscillator and inferring the capacitance of the test cell from a measurement of the frequency of the oscillator. This approach is limited by the assumption that the test cell represents an ideal or pure capacitor, failing to take into account the conductance of a real capacitor, also present in the test cell. Thus, some degree of error is inherent in this method of measurement. Also, the real capacitor of the test cell has a complex admittance comprising a capacitive component and a frequency dependent resistive component often called the loss factor. Thus, the above measurement also fails to take into account the frequency dependent resistive component.
The dielectric constant of a material causes a change in the electrical properties of a capacitor when the material is introduced in its field region, compared to the properties of the same capacitor when its field region is devoid of material. In general, the dielectric constant of a material is a function of the physical properties or constituents of the material. Thus the dielectric constant and therefore the properties or constituents of a material may be derived by introducing the material into a test cell constructed as a capacitor and measuring the change in voltage across the test cell as compared to the voltage across the empty test cell with the same signal applied thereto. If the test cell is included in an electrical network in which other components are of known fixed values and a known signal is applied to the network, voltage or gain measurements taken across the test cell filled with the sample material can be used to calculate the dielectric constant of the material. It can be shown that the conductance of the test cell capacitor with a material of known bulk conductivity therein is proportional to the capacitance of the evacuated capacitor. Thus, it is possible to eliminate the effects of the conductance and allow for the effects of both the capacitive and the frequency dependent resistive components of the real capacitor comprising the test cell by correlating separate voltage measurements taken across the test cell when empty and when filled with the sample, with signals of at least two different frequencies applied to the test cell.
Since grain moisture is defined as a percentage by weight of moisture, it has been necessary in previous moisture testing apparatus to first weigh a sample and then introduce the sample into the test cell. A preliminary moisture reading may then be obtained on the instrument either by use of a properly calibrated meter or calculating moisture from a readout on the instrument in conjunction with a chart. After this preliminary moisture calculation has been obtained, however, it is necessary to apply a correction factor for the temperature of the sample. Thus, it is necessary to measure the temperature of the sample and by reference to a suitable table or chart obtain a temperature corrected moisture reading. Also, in the case of farm grains, the moisture reading must be corrected in accordance with the variance of the bulk density of the sample from a standard bulk density. Thus, it is necessary to determine the volume of the sample being tested, in addition to its weight, to determine the density thereof. Then an additional calculation must be made, or chart referred to, to obtain a moisture content reading corrected for bulk density.
It can be seen that the foregoing procedures are relatively cumbersome when carried out with prior art apparatus due to the inconvenience of performing multiple separate measuring steps, in order to obtain a value of moisture content as corrected for bulk density and temperature. Also, it is inconvenient to perform several calculations and/or to refer to charts or tables to derive this final corrected value of moisture content.